1. Field of the Invention
The present invention relates to the field of measuring characteristic parameters of data channels for data handling devices such as data storage devices and data communications devices, and in particular to the field of measuring a characteristic pulse width for such a data channel relative to a reference pulse width. More particularly, the present invention has application to measuring the 50 percent pulse width (PW50) of a data retrieval channel of a hard disk drive (HDD).
2. Description of the Prior Art
Data handling devices, such as data receivers communicating with data transmitters or mass storage devices that retrieve data from mass storage, typically receive data signals from a data source and process them for further use. The characteristics of the data channel provided by the data handling device present limitations on the density with which data can be, for example, stored or transmitted. In particular, such a data channel will have a characteristic pulse width that determines a maximum data density in the subject data signal that the device can process without producing an excessive rate of errors.
This disclosure principally addresses the particular case of characteristic pulse widths for data channels in hard disk drives. However, as persons of ordinary skill in the data signal processing arts will recognize, the present invention has application to a broad range of data handling environments where hardware limitations present limitations on the data density of the data signals that the data handling device can adequately process.
A computer system with an attached HDD typically uses the HDD as a large capacity auxiliary storage device. A suitable HDD for such an application comprises a magnetic disk having a solid substrate of, for example, aluminum on which a magnetic material is coated. A magnetic head records data on the magnetic disk. The HDD also uses the magnetic head to detect and reproduce data signals stored on the magnetic disk. Once the data signal is detected and reproduced, the desired data must be extracted from it. The sophistication of the extraction procedure plays a large role in the maximum data density allowed by the data channel: a more sophisticated extraction procedure permits higher data densities because it more reliably differentiates between closely-spaced data pulses in the data signal.
The peak detector method, a successful technique for data extraction, has been widely used to enhance the permissible density of data recorded on the magnetic disk. In this and similar data recovery methods, noise inevitably is generated as a consequence of inter-symbol interference (ISI). This noise manifests itself in errors that occur when data recorded on the magnetic disk is reproduced. This tendency toward error generation constrains the degree to which the density of data recorded on the magnetic disk can be enhanced.
In order to further enhance data storage densities in HDDs, various alternative techniques have developed to replace the peak detector method. One class of such techniques includes the partial response maximum likelihood (PRML) methods, including PR4, EPR4 and E.sup.2 PR4. These methods provide enhanced data recovery by utilizing mutual calculations of sampled symbols to offset inter-symbol interference. ISI thus does not constitute a limiting factor for PRML methods on the enhancement of recording density on a magnetic disk.
On the other hand, in PRML methods the so-called user density does become an important limiting factor for enhancement of recording density. User density is defined as the ratio of the PW50 value to the user bit length: EQU USER DENSITY=PW50/USER BIT LENGTH (I)
In equation (I), PW50 denotes the pulse width at 50 percent of peak amplitude, which is determined by the characteristics of the magnetic head and disk. The user bit length is defined as the duration of a one-bit data signal on the time axis and has the dimensions of time/data bit. A corresponding data rate may be defined as 1/user bit length and carries the dimensions of data bits/time.
Different data recovery methods, such as the peak detector method as against PR4, generally have different error rates for a given user density. Generally, though, for a given data recovery method the error rate in recovered data increases as a monotonic function of the user density. Each such method therefore has an associated maximum allowable user density, because the user density must be maintained below a specific value in order to maintain the error rate from the method within an acceptable upper limit such as 10.sup.-11. User density therefore constitutes a fundamental limiting factor for all data recovery methods, including the PRML methods, because acceptable performance of the HDD requires that the user density not induce a higher than acceptable error rate.
In this connection, continuous development of the PRML methods, such as PR4, EPR4, and E.sup.2 PR4, has allowed increased user densities while maintaining error rates within acceptable limits. These developments have yielded increases in the data recording densities available from HDDs. At the level of implementation, an increase in user density is achieved by reducing the user bit length for a given PW50 value through software adjustment of the data write rate relative to the angular speed of the magnetic disk, as is well known in the art.
The PW50 value, on the other hand, does not admit of easy adjustment because it is substantially determined by the physical characteristics of the magnetic head and disk in the HDD. A particular feature of characteristic pulse widths for an HDD, such as the PW50 value, is that the pulse width has different values for head positions in different regions of the disk, and it increases as the head moves toward the center of the disk. This relationship is an inherent consequence of the detector hardware in the head and of the fact that, for a constant angular speed of the disk, the linear speed of the disk surface is slower near the axis of rotation than near the disk edge.
It follows that if the HDD uses a constant user bit length over the entire disk, then the local user density for regions near the disk center will be less than for regions near the edge. Conversely, in order to design the HDD so as to maintain the user density roughly constant over the entire area of the magnetic disk, larger user bit lengths must be used in regions of the magnetic disk closer to the disk center. This criterion is equivalent to requiring that the data rate be reduced as the magnetic head moves toward the center of the magnetic disk, because the data rate is just the reciprocal of the user bit length.
An HDD that makes efficient use of the storage area on the magnetic disk therefore must allow the data rate to be varied at least discontinuously across the surface of the disk. This variation is necessary so that the data rate in use can decrease as the magnetic head moves toward the disk center. Generally the HDD design will specify a piecewise constant data rate: the disk is divided into concentric bands and a constant data rate is applied for each band, with the data rate for each band lower than the data rate for its outer adjacent band.
A key design issue in specifying a piecewise constant data rate for an HDD is where and by how much to make the data rate transitions, i.e., where and how wide the bands should be and by how much the data rate should change between bands. If low data rates are used for a large area of the disk, such as when the bands are relatively wide or data rate transitions are clustered near the disk edge, then the user density for some portions of the disk necessarily will be relatively low. This fact follows from the requirement that the user bit length at every position on the disk cannot be smaller than the minimum value that maintains the local user density below the value that corresponds to the maximum allowable error rate. If the data rate is substantially lower than this minimum value over large areas of the disk, then the overall data recording density will be reduced and, commensurately, the amount of data recordable on the magnetic disk will be reduced.
Thus the efficient design of an HDD includes selecting data rate change points to maintain the recording density as high as possible while maintaining the error rate below the maximum acceptable value. This criterion implicates the PW50 value because the local minimum value for the user bit length depends, through equation (1) and the relationship between user density and error rate, on the local PW50 value. It also complicates selecting a suitable set of data rate change points, because the characteristic pulse width for a data channel provided by a given magnetic head depends on the intrinsic physical characteristics of the head. In particular, the PW50 value at a given point on the disk typically varies significantly, not only from one head design to another, but also from one specific head unit to another. Thus, when a first magnetic head has a PW50 value larger than that of a second head, the HDD design should allow more data rate change points to be used with the first head, in order to provide the same average user density.
Conventionally, the variability in PW50 characteristics among a plurality of magnetic heads has not been treated as a critical issue. Recently, though, in view of the need to increase the recording densities of HDDs, this variability has required data rate change points to be adjusted individually for each magnetic head. A widely-used method for this process has been termed adaptive zone optimization (AZO).
The AZO method adjusts the data rate change points in accordance with the PW50 characteristics of a given head, but it does so only indirectly. The PW50 pulse width for a typical HDD has a value of only tens of nanoseconds. Direct measurement of such intervals in the context of HDD circuitry requires expensive, specialized equipment. Thus, existing implementations of AZO do not measure PW50 values directly, but instead measure the error rates generated by a candidate set of data rate change points. These measurements are typically made for a given band having a constant data rate at the highest user density portion (innermost portion) of the band.
This typical approach to AZO has an inherent limitation because the desired error rate for an HDD typically falls within the range of 10.sup.-12 -10.sup.14. Generating a sufficiently reliable error rate estimate in this range would require several days of testing under normal use conditions, and in such a case the efficiency of AZO would be very low. Thus, in order to reduce the time required to measure the error rate for each zone in an HDD, AZO typically includes application of a stress, such as an offtrack stress, to the HDD to increase the error generation frequency. This enables the AZO method to generate an approximate value of the error rate in an acceptably short testing period. On the other hand, application of such stresses may produce unexpected effects on the performance of the HDD and thereby degrade the reliability of the error rate estimate.
I have therefore found that a need exists for a way to optimize the performance of data handling devices by directly measuring the pulse width characteristics of data channels in such devices. Such an invention would enable implementation of an improved AZO method for selecting data rate change points in HDDs. It would also have direct application in a broad range of data handling devices for which performance must be optimized with regard for the variability of individual hardware components and their characteristics that determine the characteristic data channel pulse width. Such an invention should provide an inexpensive, rapid, and reliable means to measure the characteristic pulse width of individual data handling devices. Preferably it would require little additional hardware to implement in a production environment. Ideally, it could be integrated into the HDD device itself.